[Qt-interest] lazy evaluation of QListWidgetItem decorations?

[Andre Somers]

On 9-6-2010 18:29, Paul Miller wrote:
> I'm using a QListWidget and QListWidgetItems that override data() to
> return a custom DecorationRole. In this case, they are icons, and
> getting them can be expensive.
>
> I have several of these QListWidgets in a QTabWidget, and unfortunately
> the QListWidgets in hidden tabs are still calling data() to get the
> decoration. I only want this to happen on the QListWidgetItems that are
> ACTUALLY visible. It should wait until the user selects one of these
> hidden tabs to actually attempt to get the decoration.
>
> Is it possible to set this up to do this?
>
>    
Problem is, that your decorations may have different sizes, and thus 
impact the layout of the view. A QAbstractItemView-derived class is not 
much more than a component to manage the layout of items, which are 
rendered by a QAbstractItemDelegate-derived class. So, in order to know 
these sizes, it needs to retreive the data on the images, unless...

1) you tell the QListView (QListWidget is derived from that class) that 
all items have the same size using setUniformRowHeights. In that case it 
will only query the height of the first item, and use that for all other 
items; or
2) you not only reimplement data to return a decoration, but also a size 
hint. If a size hint is supplied by the model (or the QListWidgetItem, 
which will supply it to the model in turn), then that size hint will be 
used for the layout calculations.

Alternatively, you could choose not to actually populate the lists 
before showing them. At least that will save them from being layed out 
needlessly.

HTH,

André
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